The present invention relates generally to rotating labyrinth seals and particularly to rotating labyrinth seals having improved machinability for use in gas turbine engines for the propulsion of aircraft.
Rotating labyrinth seals have a wide variety of uses and one such use is to effect sealing between plenums at different pressures in gas turbine engines. Such seals generally consist of two principal elements, i.e., a rotating seal and a static seal or shroud. The rotating seal, in cross section parallel to the axial length of the engine, frequently has rows of thin tooth-like projections extending radially from a relatively thicker base toward the static seal or shroud. The static seal or shroud is normally comprised of a thin honeycomb ribbon configuration for jet engine applications. These principal elements are generally situated circumferentially about the axial (lengthwise) dimension of the engine and are positioned with a small radial gap therebetween to permit assembly of the rotating and static components. The purpose of the labyrinth seal arrangement is to minimize gas path leakage out of the primary gas path and to segregate different stages of the compressor which are at different temperatures and pressures.
To a significant extent, engine efficiency depends upon minimizing this gas leakage around rotating components by controlling the gas flow to maximize interaction between the gas stream and the components in the primary gas path. The effectiveness of the turbine engine varies directly with the proportion of gas that impinges upon the blades of the rotating member. Closer tolerances between the rotating and static seals achieve greater efficiencies. The fabrication process to obtain these close tolerances is extremely costly and time-consuming.
When the gas turbine engine is operated, the elevated temperatures of operation cause the opposed static and rotating seals, such as those in the rotating labyrinth seals, to expand in a radial direction toward each other. The rotating labyrinth seals expand radially and rub into the shroud, creating frictional contact between the thin projections of the rotating seal and the shroud. During the rub, there is high thermal compression, with resultant high residual tensile stress after the rub. This frictional contact causes elevation of seal teeth temperatures in excess of 2,000 degrees F. with resulting possible damage to one or both seal members. For example, rotating tips may crack and break off, significantly impairing the seal efficiency and operation of the engine.
The thin, honeycomb ribbon construction of the shroud is used to reduce the surface area on which the seal teeth rub while reducing the weight of the structure, and helps to minimize the heat transferred into the rotating seal, while also providing the required strength. In addition, the rotating labyrinth seal teeth tips are made thin in order to thermally isolate them from the supporting base or shell structure. However, excessive heat from deep rubs (even into honeycomb) during engine start-up and during engine excursions can damage the rotating knife edge seals, negatively affecting durability and engine efficiency and providing a leak path for the flow of gases. Furthermore, material transfer can occur which also degrades the seal characteristics. Cutting into even low-density honeycomb cells can still cause rotary seal tooth damage, leading to premature part retirement.
Various coating techniques, for example, U.S. Pat. No. 5,314,304 to Wiebe, have been employed to coat the inside diameter of the stator shroud with an abradable coating in an attempt to increase both service life and operating efficiencies. The abradable coating can be worn away by the frictional contact of the rotating seal, thereby providing a close fitting channel in which the rotating seal may travel and maintaining efficiencies. One problem with an abradable coating system is that, over time, the abradable material filling the cells of honeycomb can separate from the substrate honeycomb as a result of thermal cycling. The abradable filler can then rub against the downstream blades causing engine vibrations. Other problems include inadequate sealing, seizing of cooperating members, elevation of the temperature of the rotor teeth as a result of the frictional wear induced by contact with the abradable coating and local xe2x80x9chot-spotsxe2x80x9d with resulting burning of non-abradable members.
Kobayashi et al. in U.S. Pat. No. 6,039,535 also addressed the problem of performance of the labyrinth seal for a centrifugal compressor. An abradable coating is applied over the casing. The improved seal is formed by bonding an additional layer of abradable material over the substrate material of the casing. The compressor clearances are set so that the tips of the rotating seal do not contact the substrate material casing, but rather only contact the abradable coating over the casing. The overall thickness of the casing and coating is increased to fill the gap between the substrate material casing and the rotating labyrinth seal. Kobayashi et al., however, does not disclose the use of a thin, ribbon like honeycomb material for a casing, and hence does not recognize the problems associated with forming a honeycomb material from thin ductile sheets and subsequently applying an abradable coating over the honeycomb material.
Other attempts at increasing engine efficiencies have included coating the seal teeth. For example, U.S. Pat. No. 5,603,603 to Benoit et al. is directed to applying an abrasive tip coating to the seal teeth, and U.S. Pat. No. 4,884,820 to Jackson et al. is directed to bonding a ceramic or metallic coating to the seal teeth.
Another approach addressing rotating seal tooth durability has been to make the seal teeth more defect tolerant, such that cracks that form due to rubbing are benign U.S. Pat. No. 5,143,383 to Glynn et al. relates to stepping the tooth profile to act as a crack arrestor. This method has a disadvantage of being dependent on a relatively low mean stress and stress range to avoid having crack growth of critical size during the expected life of the typical gas turbine part.
U.S. Pat. No. 4,060,250 to Davis et al. is directed to non-aircraft centrifugal compressors, in which the carbon steel rotary elements are inlaid or coated with a corrosion and heat resistant alloy, such as a chromium-containing nickel-based alloy, added to protect the underlying low carbon steel from ignition. The surface of the rotatable cylindrical member is characterized by this metallurgically fused alloy protective coating.
While much effort has been directed at improving the rotating structure of the seal arrangement, There is a continuous need for improved designs for rotating labyrinth seal structures including improvements directed to the static structure to increase both service life and engine operating efficiencies. The present invention fulfills this need, and further provides related advantages.
The present invention provides for a method to increase the machinability of the honeycomb of the stationary portion of a labyrinth seal. The increased machinability of the stationary portion of the labyrinth seal, referred to as a shroud, results in a reduction in the measured peak tooth temperature of the rotating seal teeth of the rotating portion of the labyrinth seal, while maintaining or even improving the high temperature capability of the rotating labyrinth seal, so as not to limit its operating environment. The sealing functionality of the rotating labyrinth seal is unaffected, and even improved in some instances, by the method of the present invention. The improved machinability of the shroud results in less friction between the shroud and the rotating teeth, thereby reducing damage to the teeth.
After forming a honeycomb, which will be used as a seal or shroud, from a thin ribbon of ductile substrate material such as a superalloy material, the machinability of the honeycomb is increased by selecting a lightweight diffusible element that is capable of affecting the ductility of the substrate material by making it more frangible, that is to say, causing it to fracture into small particles in a machining event. The lightweight, diffusible element is diffused at a preselected, elevated temperature into the honeycomb ribbon substrate to cause a coating to be formed at or below the surface of the substrate. This coating causes the surface of the honeycomb to have mechanical properties that are different from the unaffected and underlying substrate portion of the honeycomb. The coating imparts improved machinability to that portion of the substrate into which it is grown, while also reducing the ductility of the substrate surface. The coating grown into the surface of the substrate has an effective, preselected depth that is thin. However, the thin coating is frangible, while the underlying substrate of the shroud remains ductile. Thus, the outer portion of the honeycomb seal having the thin coating has different machining characteristics than the underlying base material substrate of the shroud.
The present invention also includes the stator honeycomb shroud produced by the foregoing method of growing a diffusion coating into the substrate surface.
One advantage of the present invention is that the improved machinability of the honeycomb results in a reduced labyrinth rotor seal tooth temperature during the rub of the rotating labyrinth seal into the honeycomb shroud. Reduced temperature leads to reduced damage to the seal tooth, thus reducing the propensity to crack and hence propagate, and thereby increasing the service life.
Another advantage of the present invention is the reduced torque resistance. Because less torque is produced during the rub, simpler rotor designs may be produced, since they no longer require anti-rotation features. This is because friction alone on rabbets, which are radial contacts in a rotor assembly that maintain concentricity of the rotor, is adequate to maintain the rotating seal in position during rubs.
Still another advantage is the reduced metal transfer between honeycomb shroud and tooth tip, highly beneficial to maintaining seal clearances. The metal transfer is generally local in nature, causing a local high spot on the rotor which will then become the only place future rubs occur. This has the negative effect of leaving the remainder of the circumference with a larger gap as the local high spot on the rotor removes material substantially uniformly from the stator, resulting in greater engine inefficiencies. Less metal transfer results in a smaller gap and greater engine efficiencies.
Yet another advantage is that environmental resistance can be maintained or selectively improved for assuring that part life will not be shortened as a result of the improved machinability.
Another advantage is the reduction in the tendency of the rotating labyrinth seal to go out-of-round. Because rotating labyrinth seals are rarely manufactured perfectly concentric with the stator, local rubs are the norm. Out-of-roundness of the rotating seal results from local rubbing against the corresponding, adjacent shroud and subsequent sinusoidal thermal gradients which occur due to local rub. This out-of-roundness can result in deeper rubs into the shroud, resulting in even larger post-rub clearances. These deeper rubs can provide an increase in the amount of heat input into the teeth, resulting in damage or ultimate failure of the teeth. The decreased tooth heating of the present invention will reduce the tendency of the local rubs to cause deeper rubs and hence reduce the tendency of the seal to have larger post-rub clearances as the out-of-round conditions are minimized. Damage to the rotating seal teeth is also reduced.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying figures which illustrate, by way of example, the principles of the invention.